The primary purpose of the lungs is to transfer oxygen into and carbon dioxide out from the blood. Diagnostic testing for lung disease is called pulmonary function testing. Most commonly, this testing is done on three categories of lung function, the mechanics of the lungs or the ability of the lungs to move air in and out, the measurement of lung volumes and the testing for the ability of the lungs to transfer oxygen into and carbon dioxide out from the blood, or diffusing capacity. Measuring the diffusing capacity for oxygen is difficult because of the inability to measure the mean partial pressure of oxygen in the pulmonary capillaries. Carbon monoxide (CO) on the other hand diffuses rapidly into the blood and attaches to the hemoglobin, which has a great affinity for CO. Because of this affinity, the test commonly employed uses CO and is called the diffusing capacity test for carbon monoxide or DLCO.
DLCO requires equipment designed to perform the test according to specifications of the American Thoracic Society (ATS); the latest version of this specification was adopted March 1987. This equipment must provide a means for a measured volume of test gas comprised of CO, air and an inert gas such as helium to be inhaled by the patient. The gas is held in the lung for 10 seconds and then exhaled. CO and the helium concentrations are known or measured before and measured after breathhold. The CO concentration is reduced by the amount diffused into the blood. The helium does not diffuse, but is diluted by the residual volume of the lung and thus reduced in concentration. Other inert gases may be used such as argon.
In the prior art the DLCO equipment has divided the expired sample into three parts. The first part, 750 ml, is discarded because it comes from the anatomical dead space in the lung. The next 500 ml to 1000 ml is collected in a sample collection bag. The remainder of the exhaled gas is also discarded. The sample collected in the bag is then transferred to gas analyzers, one for CO and one for helium. Scrubbers to remove water vapor and carbon dioxide (CO.sub.2) as interferences in the gas analyzers may also be used.
The measurement of the mechanics of the lung is done with a volume spirometer or pneumotach. Lung volumes can be measured with various equipment. Spirometry has been known and used in some forms in diagnosis of lung disorders for a long time. However, what can be learned from spirometry alone is limited. Measurement of pulmonary diffusing capacity has been understood since about 1915, but was not a practical tool for many years. Equipment for clinical use of pulmonary diffusing capacity was developed in the 1950's. Since that time, such equipment has been used in hospital pulmonary function labs to assist in the differential diagnosis of lung disease. Automatic valve sequencers developed early in the 1970's made differential CO measurements less difficult and more reproducible. These sequencers made handling of the inhaled gas and the exhaled sample more automatic. However, the same pieces of equipment for gas analysis and scrubbing are still required by equipment today.
Interstitial lung diseases, such as asbestosis, coal workers pneumoconiosis, and silicosis, caused by the inhalation of inorganic dusts, can be diagnosed by such equipment. Annual spirometry is now required by law for workers exposed to those enumerated and many other diseases produced by inorganic dusts. However, unavailability of portable, easy-to-use devices for testing diffusion capacity has made impractical enactment of the legislation requiring use of such devices.
It is widely recognized that clinical screening of patients with interstitial lung disease would be enhanced by diffusion capacity testing. Such testing would much more readily demonstrate a significantly impaired gas exchange in many patients who have no significant defect in the mechanics of breathing, as indicated by the spirometer. Furthermore, many of these patients have normal chest X-rays. Therefore, testing for diffusion capacity of the lungs for carbon monoxide is the only objective evidence of lung disease. Such testing is rapid and noninvasive. If the equipment were more readily available, especially in the field, far better testing for lung disease could be accomplished.
The long time availability of equipment for determining diffusion capacity for carbon monoxide has been limited to special clinical installations because heretofore the equipment has been heavy and bulky, complex and usually quite expensive. Also, prior equipment has often required the collection of a sample of expired gas followed by the transfer of the sample or parts thereof to discreet gas analysis equipment, one for carbon monoxide and one for helium. Transfer of the sample has required a pump or some other motor driven device, although it could be done by a hand syringe. The various steps take technician time. It is alternatively possible to use a gas chromatograph, an additional expensive equipment, to identify the gases. Therefore, these techniques have been too expensive for general use. In addition, the involvement of many mechanical and electronic components has increased the potential for malfunction. This same complexity has tended to make the equipment heavy so that, as a practical matter, it must be used at one place.